The present invention relates to apparatus for mixing a plurality of gases and in particular apparatus for mixing a plurality of medical gases and forming part of an anaesthesia machine.
Anaesthesia machines of the continuous flow type are the most widely used today and are employed in conjunction with a breathing circuit to provide a complete anaesthesia system. Such machines blend or mix a gaseous anesthetic with oxygen in proportional amounts to produce a gaseous mixture having a desired flow rate. A typical example of a continuous anaesthesia machine is disclosed in U.S. Pat. No. 3,739,799 which includes at least two flowmeters operable to measure and visually indicate the rate of flow of respective gaseous components. A mixture control valve is operable to increase the rate of flow of either of said components to its associated flowmeter and simultaneously decrease the rate of flow of the other of said components to its associated flowmeter to vary the relative proportions of said components substantially without varying the total flow of said components. A total flow control means is operable to vary the total rate of flow of the said components substantially without varying the relative proportions of said components.
This known anaesthesia machine has proved to be highly successful but suffers from the disadvantage that the mixture control valve is difficult to manufacture and calibrate. The mixture control valve comprises a pair of needle valves extending axially outwardly from opposite sides of a calibrated disc or dial. One of the needle valves is provided with a threaded portion received in a threaded bore for axially moving the needle valves relative to ports surrounded by valve seatings. The distal ends of the needle valves are each provided with a tapered surface which is adapted to co-operate with its respective seating to close or open a port when the mixture control dial is turned. When the dial is turned in one direction the axial movement of the needle valves causes one valve to distance itself from its seating whilst concomitantly allowing the other needle valve to approach its seating.
In this manner increasing the concentration of one gaseous component and the individual flowrate thereof reduces the concentration of the other gaseous component and the flowrate thereof by an equal amount thereby varying the proportions without effecting the total flow rate.